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Old Jan 25, 2013, 09:16 AM
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France, PACA, Aix-en-Provence
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Here in France we get very high temperature gradients between day and night and my wings have been wet to the touch when I slid them in their wingbags... very annoying.

True, I think condensation may play a role, however my logic would suggest that this would be applicable to the exterior of the servo housing due to the volume of air. Internal to the servo there is a small volume of air and I would assume it would follow the temperature of the case surrounding it very quickly, reducing/removing any condensation effects. This is not the case for the air around the servo, which is not modified by the metal case temperature and this will cause the condensation to occur. This is just me making assumptions though...


http://www.hyperflight.co.uk sells MKS as well; quick and easy.
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Old Jan 25, 2013, 10:00 AM
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Essex UK
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the problem I have at the moment is getting hold of replacement gear sets :-(
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Old Jan 25, 2013, 12:08 PM
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Chippenham, Wilts
Joined Oct 2005
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Originally Posted by Woodstock 1 View Post
Jokes aside, if I were living in the UK (or most northern European Countries) and competing in F3F or other races through the winter (as many do), I would be very cautious about using servos with a metal case. The planes are subject to numerous sudden extreme climate changes as they are moved from cold cars or sheds into warm, heated cars, then out into extreme cold on the slope, back into warm cars, and back into cold sheds, etc. I would think the risk of internal condensation within the servo case is higher than with plastic cases...

Chris
Agreed Chris. I have seen 2 mks wing servos play up in the cold weather and both eventually failed completely
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Old Jan 25, 2013, 12:44 PM
Hmmm Nice!
bags's Avatar
Joined Sep 2004
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Originally Posted by jiberjaber View Post
the problem I have at the moment is getting hold of replacement gear sets :-(
http://www.t9hobbysport.com/item.asp...170&prodid=837
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Old Jan 25, 2013, 12:55 PM
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Originally Posted by rumbey View Post
Agreed Chris. I have seen 2 mks wing servos play up in the cold weather and both eventually failed completely
Very interesting. When people mention about issues, they tend to have more than one. Weather conditions could play a part. This is something worth keeping an eye on.
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Old Feb 03, 2013, 03:52 AM
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Thanks Rich - somehow I missed this post but I ordered them yesterday so should be able to get on with the refurb when they arrive TA
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Old Feb 03, 2013, 06:48 AM
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France, PACA, Aix-en-Provence
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Urgh, I think I spoke to soon.

Went out flying today and noticed some noises on my approach to final. One of my flap servo's (MKS 6125 Mini) goes nuts around a particular place of motion, around half brake deployment. It can oscillate quite wildly if I leave it in that position. I took a video, but need to figure out how to post it.

About the cold/moisture issue, my wings sleep in the garage, my fuses in the kitchen cupboard... And it was pretty cold today. So this could be a factor.

So that is 1 out of 8 servo's with an issue :-(
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Old Feb 03, 2013, 07:01 AM
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If you upload your video to Youtoob or Vimeo, then you just copy and paste the web address of that video to your post....

Chris
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Old Feb 03, 2013, 07:52 AM
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Video: MKS 6125 woes

Please note that the agressive wobble towards the end is not controlled...
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Old Feb 03, 2013, 10:27 AM
Fit to CFIT
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Thousand Oaks, CA
Joined Mar 2004
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That's perfectly normal Belgin. It's just simple resonance and will occur with any servo when the flap mass and linkage stiffness/geometry form a resonant frequency close to that of the servo. It happens in one spot because the linkage geometry changes thru the range, effectively tuning the resonant frequency.

Put the flap at that angle and slap the wing. It will start resonating. Now touch the flap and it will stop.

The cure is to stiffen the system or lighten the flap. It looks like you could benefit from a shorter servo arm which should cure the problem.
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Old Feb 03, 2013, 10:39 AM
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Originally Posted by vespa View Post
That's perfectly normal Belgin. It's just simple resonance and will occur with any servo when the flap mass and linkage stiffness/geometry form a resonant frequency close to that of the servo. It happens in one spot because the linkage geometry changes thru the range, effectively tuning the resonant frequency.

Put the flap at that angle and slap the wing. It will start resonating. Now touch the flap and it will stop.

The cure is to stiffen the system or lighten the flap. It looks like you could benefit from a shorter servo arm which should cure the problem.
Ha ha, good one !
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Old Feb 03, 2013, 10:52 AM
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France, PACA, Aix-en-Provence
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Originally Posted by vespa View Post
That's perfectly normal Belgin. It's just simple resonance and will occur with any servo when the flap mass and linkage stiffness/geometry form a resonant frequency close to that of the servo. It happens in one spot because the linkage geometry changes thru the range, effectively tuning the resonant frequency.

Put the flap at that angle and slap the wing. It will start resonating. Now touch the flap and it will stop.

The cure is to stiffen the system or lighten the flap. It looks like you could benefit from a shorter servo arm which should cure the problem.
I bypassed the possibility of this and took the servo out of the wing.
It moves perfectly smoothly throughout the entire range.
When I put resistance on it (when it is stationary or moving) with my fingers it does the same oscillating motion. This motion also stops instantly when I let go of the servo. So unless the resonance frequency of my fingers matches that of my flap, there is something wrong with the servo

I have to add that I have not abused this servo (yet). It is in a pretty new glider with about 10 flights on it and no landings that were overly abusive, and I always pull the flaps back prior to landing.

For the lenth of the servohorn, it is about as short as can be to get the full range of motion (with maxed out travel on the 8fg I get about 10 degrees up and about 75 degrees down). Do you think I can get more travel?
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Old Feb 03, 2013, 11:43 AM
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Thousand Oaks, CA
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Interesting. Yeah, that's definitely a bad servo then. Surprising that it would behave that way. Usually when a potentiometer problem causes severe jitter it occurs only when the load is light or absent. I've never seen one jitter only under load. Does it occur only in one position?
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Old Feb 03, 2013, 12:05 PM
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Yeah, it's got me baffled as well.

It does not seem to matter what position it is in. As soon as I put a load on it (it does not take much), it starts oscillating...
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Old Feb 03, 2013, 01:06 PM
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near Sacramento, CA
Joined Aug 2010
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Sounds to me like it's a servo problem that occurs when the servo motor draws any substantial current. This could be caused by a flaky solder connection in the servo motor or wires or on the electronics board.

Couple of things to try: Add your finger load and then try moving the servo v-e-r-y slowly (1 click at a time) and see if the same jumpiness happens. It may be that very slow movement and lower motor current might allow it to move smoothly. In any case it's still an internal servo problem.

The other thing you could try is heating up and cooling down the servo case to see if the problem is affected by temperature. If so that could point to an internal circuit board or IC defect. To heat up the case you could use a heat gun at the low setting and get the case very warm but not hot to the touch. To cool it down if you don't have a cold spray can you could use an ice cube in a plastic bag and press it against the servo case for 10 minutes. If heat or cold affects the problem it also points to an internal problem.

At this point I would ask MKS for a new servo to try since I think Vespa's call on this is right: sounds like a bad one.

Chris B.
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